With their bright red, hairless chests and grass-grazing lifestyle, gelada monkeys are quite unusual. They are the only primate, other than humans, to primarily live on land instead of in trees, and a new study shows they are also able to detect emotional and social cues through vocal exchanges.
“Geladas are special because they live in large, tolerant social groups and rely heavily on vocal communication to interact with one another. They don’t just make noise — they ‘talk’ using a variety of sounds in different emotional and social contexts,” study co-author Luca Pedruzzi told Mongabay by email.
Geladas (Theropithecus gelada), which are endemic to Ethiopia, have a vocal repertoire richer than other closely related species, Pedruzzi said. Such vocal signals are used to “maintain bonds, calm tensions and respond to group dynamics,” which makes them ideal for a study looking into the meaning of vocal cues.
For the study, the researchers recorded vocalizations of geladas living in captivity at the NaturZoo Rheine in Germany in 2023. They then played those recordings for wild geladas in the central highlands of Ethiopia.
Ten adult males were exposed to four variations of vocalization from the unfamiliar captive geladas. The first type was a scream-grunt series, the usual sequence of a distress call, followed by a comforting call. The other version was reversed, a grunt series followed by a scream. For the next two, a scream was followed by a moan, which is an even stronger comforting message, and then the reverse, a moan-scream sequence.
“Geladas reacted more strongly when they heard a vocal sequence that didn’t follow the expected order — a comforting call followed by a distress scream. That’s like hearing someone say, ‘It’s okay!’ and then hearing a scream — it feels off. Their reaction suggests they noticed this violation of social logic,” Pedruzzi said. Reactions included looking in the direction of the noise, which was played on a speaker. They also paused while feeding as evidenced by hand or head gestures. The length of response was also recorded.
Pedruzzi said the geladas also responded more to moans than to grunts, which means they recognized variations in the emotional charge of a call. “This matters because it shows a level of emotional perception in primate vocal communication that we usually associate with humans,” he said.
“That kind of ‘social listening’ is rare and valuable. It helps animals navigate complex group life without always being directly involved. For us, it opens a window into how vocal communication and social awareness evolved,” he said.
“If geladas can follow vocal conversations and grasp the social meaning behind them … it suggests that some of the mental tools needed for human communication already existed in our primate ancestors,” Pedruzzi said, adding that it might be “something our species may have inherited from common ancestors with monkeys like geladas.”
Banner image of a gelada by Charles J. Sharp via Wikimedia Commons (CC BY-SA 4.0).
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